Monthly gases concentrations (in ppb) of SO2, HNO3, NH3, NO2 and O3 are measured using passive sampling techniques. Samplers are installed at all stations, mesh-side down, on the underside of a stainless steel support to avoid direct impact from wind and splashing from precipitation. Passive sampling techniques use the property of molecular diffusion of gases into the sampling device where they are collected on an impregnated filter or adsorbent material. The advantages of passive sampling techniques are that they are generally easy to carry out and low cost compared to active sampling techniques to achieve mean or integrated-time concentrations. The WMO-GAW report 122 reports on passive samplers for atmospheric chemistry measurements and their role in GAW.
Samplers are exposed at about 1.5–3 m above the ground using duplicates for each gas. The INDAAF passive samplers were developed and prepared at the Laboratory of Aerology (LAERO) and then were sent each two months on each measurement site. Samplers were sent back to LAERO for chemical analysis using ionic chromatography (IC). The LAERO analytical service participates bi-annually to the WMO-GAW quality assurance program for soluble trace compounds measured by IC. Results are available under the reference 700106 at the following address: http://qasac-americas.org/. According to these WMO inter-comparison tests, analytical precision is estimated to be 5% or better for all ions.
Detection limits for each trace gas were calculated from field blanks and found to be 0.07±0.03 ppb for HNO3, 0.2±0.1 ppb for NO2, 0.7±0.2 ppb for NH3, 0.05±0.03 ppb for SO2, and 0.1±0.1 ppb for O3. To give an indication of the precision of this sampling technique, the covariance of all duplicate samples was calculated. The reproducibility was found to be 20%, 9.8%, 14.3%, 16.6% and 10% for HNO3, NO2, NH3, SO2 and O3 respectively.
The figure below presents the typical design of an INDAAF passive sampler.
Adon, M., Galy-Lacaux, C., Yoboué, V., Delon, C., Lacaux, J.P., Castera, P., Gardrat, E., Pienaar, J., Al Ourabi, H., Laouali, D., Diop, B., Sigha-Nkamdjou, L., Akpo, A., Tathy, J. P., Lavenu, F., Mougin, E., 2010. Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa using passive samplers. Atmos. Chem. Phys. 10, 7467–7487. https://doi.org/10.5194/acp-10-7467-2010
Ossohou, M., Galy-Lacaux, C., Yoboué, V., Hickman, J.E., Gardrat, E., Adon, M., Darras, S., Laouali, D., Akpo, A., Ouafo, M., Diop, B., Opepa, C., 2019. Trends and seasonal variability of atmospheric NO2 and HNO3 concentrations across three major African biomes inferred from long-term series of ground-based and satellite measurements. Atmos. Environ. 207, 148-166.